The D vitamins are very important agents for the control of calcium and phosphate metabolism in animals and humans, and have long been used as dietary supplements and in clinical practice to assure proper bone growth and development. It is now known that the in vivo activity of these vitamins, specifically of vitamin D.sub.2 and D.sub.3, is dependent on metabolism to hydroxylated forms. Thus, vitamin D.sub.3 undergoes two successive hydroxylation reactions in vivo, leading first to 25-hydroxyvitamin D.sub.3 and then to 1,25-dihydroxyvitamin D.sub.3 and the latter is indeed thought to be the compound responsible for the well-known beneficial effects of vitamin D.sub.3. Likewise, vitamin D.sub.2, which is commonly used as a dietary supplement, undergoes an analogous hydroxylation sequence to its active forms, being first converted to 25-hydroxyvitamin D.sub.2 (25--OH--D.sub.2) and then to 1,25-dihydroxyvitamin D.sub.2 (1,25--(OH).sub.2 D.sub.3). These facts are well established and well known in the art [see, for example, Suda et al. Biochemistry 8, 3515 (1969 ) and Jones et al. Biochemistry 14, 1250 (1975)].
Like the metabolites of the vitamin D.sub.3 series, the hydroxylated forms of vitamin D.sub.2 named above are, because of their potency and other beneficial properties, highly desirable dietary supplements, or pharmaceutical agents, for the cure or prevention of bone or related diseases, and their value and possible use is recognized in patents relating to these compounds [U.S. Letters Pat. Nos. 3,585,221 and 3,880,894].
Whereas all metabolites of vitamin D.sub.3 have been prepared by chemical synthesis, there has been but little work on the preparation of vitamin D.sub.2 metabolites. The known synthetic processes for the metabolites of the D.sub.3 -series (especially as far as they relate to the preparation of side chain hydroxylated compounds) are, of course, in general not suitable for the preparation of the corresponding vitamin D.sub.2 metabolites, since the latter are characterized by a side chain structure (i.e. presence of a double bond and an extra methyl group) which requires a different synthetic approach from that applicable to side chain hydroxylated D.sub.3 compounds.
Two compounds structurally related to 25--OH--D.sub.2 have been prepared, namely 22-dehydro-25-hydroxycholecalciferol, which may be considered a 24-desmethyl analog of 25--OH--D.sub.2 (see U.S. Pat. No. 3,786,062), and 24,25-dihydroxyvitamin D.sub.2, the 24-hydroxy-analog of 25--OH--D.sub.2 [Jones et al. Biochemistry 18, 1094 (1979)]. However, the synthetic methods proposed in these reports are not applicable to the preparation of 25--OH--D.sub.2 itself. No synthesis of the latter compound has appeared in the literature, and although there is the mention in the paper by Salmond et al. (Tetrahedron Letters, 1695-1698 (1977), see p. 1697 and footnote 11) of the successful preparation of 25--OH--D.sub.2, no information on the overall process has been published to date.